For many years, electromagnetic pickups have been utilized on musical instruments having steel strings. Such pickups have been employed with guitars, bass guitars, banjos, mandolins, and a variety of other instruments. An electromagnetic pickup for a musical instrument incorporates a magnetic structure for generating a magnetic field that intersects a portion of the instrument strings; the magnetic structure of the pickup includes at least one permanent magnet and frequently has a separate high-permeability pole piece for each string. On the other hand, some electromagnetic pickups have a single pole that spans a number of strings. The pickup has an electrical coil or coils to generate a signal which is subsequently amplified and reproduced by a speaker or other transducer as the audio output of the musical instrument. The coils are customarily disposed in encompassing relation to the magnetic core or cores. This relatively simple electromagnetic structure is fitted into a housing that may or may not be part of the magnetic structure. Whether or not a part of the magnetic structure, a principal purpose of the housing is to protect the pickup from dirt and other contaminants.
A wide variety of individual constructions have been used for electromagnetic pickups employed with musical instruments such as guitars. Frequently, the efforts of the pickup designer have been directed toward achieving an output signal from the electrical coil that is as close as possible to a faithful reproduction of the sound that would be developed by the instrument functioning as an acoustical device. This is not always the case, however; some electromagnetic pickups have been designed to give a particular distortion deemed desirable by the designer or by a musician.
For electromagnetic pickups in general, as applied to musical instruments having steel or other ferromagnetic strings, there may be a problem in obtaining an output signal of sufficient amplitude. This may be a minor problem, with modern electronic technology, because a very weak signal can often be adequately amplified. On the other hand, a reasonable output amplitude is desirable because it reduces the necessity for subsequent amplification, and thus reduces the likelihood of inadequately controlled distortion. An output signal of appreciable amplitude also aids in achieving an acceptable signal-to-noise ratio
A more pronounced problem, in most electromagnetic pickups for musical instruments has to do with the frequency response. The overall "sound" derived from the output signal is usually critical to the requirements of the musician. Some musicians want to have the output signal as close as possible to the acoustic output of the instrument, at least in theory. Others, however, want to have a distortion that is acceptable to them, one that represents their own concept or technique for interpretation of music. The frequency response characteristics of the pickup are critical in this regard. A similar situation is presented by the sound characteristic known to musicians as "sustain"; sometimes accented "sustain" is desirable in the view of the musician using the pickup and sometimes it is not.